Recent broadband communication systems have used signals in which a ratio of a peak power to an average power (PAR) is about 10 dB. In such a case, it is necessary to use a high-frequency amplifier that can transmit signals with a peak power that is at least 10 dB higher than an average transmission power at a transmitting stage.
Generally, the power efficiency of an amplifier lowers as a ratio of a peak power to an average transmission power (back-off) increases. On the other hand, eco-conscious trends have demanded reduction of the power consumption in radio systems. Particularly, it has been desired to enhance the efficiency of a high-frequency amplifier, which accounts for a large part of the power consumption of a base station device for cellular phones that needs to have a high output power of at least several tens of watts.
It has been examined to use, as a high-frequency amplifier having a high efficiency with a large back-off, a Doherty-type amplifier for a base station. In the Doherty-type amplifier, only a carrier amplifier operates in a low output power region, whereas both of a carrier amplifier and a peak amplifier operate in a high output power region.
The efficiency can be maximized at an output level at which the peak amplifier starts to operate. Thus, a higher efficiency can be achieved. Furthermore, an output level can be changed such that the efficiency is maximized by changing a ratio of saturation power levels of the carrier amplifier and the peak amplifier.
In the conventional technology, a high-frequency amplifier for a cellular phone base station generally uses an LD-MOSFET (Lateral Diffused Metal-Oxide-Semiconductor Field Effect Transistor) using a material of silicon (Si), which is a single element semiconductor, as a semiconductor device.
Recently, a compound semiconductor represented by gallium nitride, which provides a semiconductor device having a higher efficiency, has been used for semiconductor devices in order to enhance the efficiency of a Doherty-type high-frequency amplifier for a cellular phone base station. Thus, a higher efficiency has been achieved. However, there is a problem that a compound semiconductor is more expensive than a single element semiconductor.
Examples of the related art that can cope with such a problem include JP-A 2008-193720 (Patent document 1). Patent document 1 discloses a Doherty amplifier including a carrier amplifier using a GaAs FET and a peak amplifier using an LD-MOSFET in order to improve the AM-PM characteristics (output power-output phase characteristics).
In general, however, a positive electrode of a bias voltage is applied to a drain of a GaAs FET, and a negative electrode of a bias voltage is applied to a gate of the GaAs FET. Therefore, both of positive and negative polarities are required as power supplies. As a result, there is a problem that a configuration of power supply for applying bias voltages to the amplifier becomes complicated.